The unusual creep and relaxation behaviour of polypropylene

This paper focuses on experimental investigation and modelling of the unusual creep and relaxation behaviour of polypropylene. A polypropylene specimen was loaded up to three different levels of maximum strain (epsilon(max)), unloaded to various minimum stresses (sigma(min)), creep and relaxation tests were performed at seven different strain levels. In addition to investigate the effect of loading history on unloading paths, multiple creep experiments were conducted at various stress levels. It was observed that unusual creep and relaxation behaviour of polypropylene depended on a strain increment (Delta epsilon) which is the difference between the minimum (epsilon(min)) and maximum strain level (epsilon(max)). Increasing the strain increment affects the creep and relaxation behaviour on loading path. As a result, the creep and relaxation behaviours of polypropylene show non-monotonic characteristics. The viscoplasticity theory based on overstress for polymer (VBOP) model was used to model the unusual creep and relaxation behaviour of polypropylene. From the comparison between the VBOP model predictions and the experimental results, it can be seen that the VBOP model can model both the unusual creep and relaxation behaviours of a semicrystalline polymer.